1. Field of the Invention
The present invention relates to voltage-follower circuits and, in particular, to a voltage-follower circuit that generates an attenuated voltage signal that swings around a reference voltage in response to an input voltage signal.
2. Description of the Related Art
A voltage-follower is a circuit that interfaces a high-impedance circuit with a low-impedance load. One application for a voltage-follower is within the voltage to frequency converter block of a phase lock loop. The converter block converts the voltage of an input voltage signal, which represents the frequency difference between a reference signal and an oscillator output signal, into an exponential control current which is utilized to control the frequency of the oscillator.
FIG. 1 shows a schematic diagram of a portion of a conventional converter block 2. As shown in FIG. 1, converter block 2 includes a first voltage-follower stage 4 that generates an intermediate voltage signal V.sub.M at an intermediate node N.sub.M in response to an input voltage signal V.sub.IN. Voltage-follower 4, which is well-known in the art, provides a high input impedance to isolate converter block 2 from the input voltage signal V.sub.IN as well as wide bandwidth and no D.C. voltage level shift. As a result, the intermediate voltage signal V.sub.M is substantially equivalent to the input voltage signal V.sub.IN.
Converter block 2 also includes a reference stage 6 that generates a pivot voltage V.sub.P at a pivot node N.sub.P. The pivot voltage V.sub.P is typically selected to represent the approximate midpoint of the range of input voltage signals V.sub.IN. Thus, as the input voltage signal V.sub.IN varies, the input signal V.sub.IN will be greater than the pivot voltage at times, less than the pivot voltage at other times, and equivalent to the pivot voltage from time to time. Therefore, as the input voltage signal V.sub.IN varies around the pivot voltage V.sub.P, the intermediate voltage signal V.sub.M also varies around the pivot voltage V.sub.P.
Converter block 2 additionally includes a voltage divider stage 8 which is connected between the intermediate node N.sub.M and the pivot node N.sub.P and that generates an attenuated voltage signal V.sub.ATTEN in response to the difference between the intermediate voltage signal V.sub.M and the pivot voltage N.sub.P.
As shown in FIG. 11, voltage divider stage 8 includes nine series-connected resistors R14A, R14B, R14C, R14D, R14E, R14F, R14G, R14H, and R14I connected between the intermediate node N.sub.M and an attenuation node N.sub.A, and two parallel connected resistors R15A and R15B, connected between the attenuation node N.sub.A and the pivot node N.sub.P.
Voltage divider stage 8 functions as a simple voltage divider. In operation, when the intermediate voltage signal V.sub.M varies about the pivot voltage V.sub.P, an attenuated voltage signal is generated which also swings around the pivot voltage V.sub.P, but with a reduced magnitude.
In FIG. 1, for example, the pivot voltage V.sub.P is set at approximately 2.1 volts as a result of the voltages across the base-emitter junctions of transistors Q1, Q12, and Q13. When the intermediate voltage signal V.sub.M and the pivot voltage V.sub.P are equivalent, such as 2.1 volts, an equivalent attenuated voltage signal is generated at the attenuation node N.sub.A. When the intermediate voltage signal V.sub.M increases to 2.2 volts (compared to a 2.1 volt pivot voltage), the attenuated voltage signal V.sub.ATTEN increases to approximately 2.105 volts. Similarly, when the intermediate voltage signal V.sub.M decreases to 2.0 volts, the attenuated voltage signal V.sub.ATTEN decreases to 2.095 volts.
In addition, when the intermediate voltage signal V.sub.M and the pivot voltage V.sub.P are different, a divider current I.sub.D is generated which flows through the voltage divider stage. When the voltage of the intermediate voltage signal V.sub.M is greater than the pivot voltage V.sub.P, divider current I.sub.D flows away from the intermediate node N.sub.M into the pivot node N.sub.P. Similarly, when the intermediate voltage signal V.sub.M is less than the pivot voltage V.sub.P, divider current I.sub.D flows from the pivot node N.sub.P into the intermediate node N.sub.M.
Converter block 2 further includes a second voltage-follower stage 10 that generates an attenuated input voltage signal V.sub.AIV in response to the attenuated voltage signal V.sub.ATTEN. Voltage-follower 10, which is also well-known in the art, provides a high input impedance to isolate divider current I.sub.D of voltage divider stage 8 from the remainder of converter block 2 as well as wide bandwidth and no D.C. voltage level shift. As a result, the attenuated input voltage signal V.sub.AIV is substantially equivalent to the attenuated voltage signal V.sub.ATTEN.
Converter block 2 also includes a transconductance stage 12 that generates an exponential control current I.sub.C in response to the attenuated input voltage signal V.sub.AIV. As stated above, the exponential control current is utilized to control the frequency of the oscillator output signal.
As shown in FIG. I, transconductance stage 12 commonly utilizes a bipolar transistor, such as transistor Q29, as an exponential transconductance amplifier to generate the exponential control current I.sub.C. It is the high transconductance of a bipolar transistor (small changes in the base voltage produce large changes in the collector current) that necessitates the voltage gain reduction provided by voltage divider stage 8.
One problem with converter block 2 is the extensive circuitry that is required to generate the attenuated input voltage signal V.sub.AIV from the input voltage signal V.sub.IN, particularly the need for a second voltage-follower stage 10 to isolate voltage divider stage 8 from transconductance stage 12. The presence of second voltage-follower stage 10 increases both the power consumed and the circuit area required by converter block 2.
Thus, there is a need for an attenuating voltage-follower circuit that can generate an attenuated input voltage signal without the second voltage-follower stage, thereby significantly reducing both the power consumed by the converter block and the circuit area required for the converter block.